The present application relates to and incorporates herein by reference Japanese Patent Application No. 2000-102385 filed Apr. 4, 2000.
1. Field of the Invention
The present invention relates to an angular position detection device which detects an angular position of an object by use of a magnetism detection element.
2. Related Art
An angular position detector [is] has been proposed in JP-A-61-75213. This angular position detector is constructed so that a cylindrical magnet is magnetized in the radial direction, and a magnetism detection element is disposed in the middle of the magnet, and the magnet (or the magnetism detection element) rotates with rotation of an object to be detected. In this construction, when the magnet rotates, the amount of magnetic flux passing through the magnetism detection element varies and an output signal from the magnetism detection element varies with the amount of the magnetic flux. Thus the angular position of the object can be detected by means of the output signal.
When detecting the angular position of the object, by means of the output signal in this way, in order to expand the range of angular position detection, characteristics of the output variation in the magnetism detection element against the angular position should be made linear over as wide a range as possible.
However, in the above prior art, since the parallel magnetic field is rotated about the magnetism detection element, the magnetism detection directional component of the magnetic flux passing through the magnetism detection element decreases in trigonometric function as the angular position advances. As a result, the characteristics of the output variation in the magnetism detection element against the angular position deviates from the straight line and curves in trigonometric function. Therefore the prior art has a problem that it provides only a pseudo-linear output over a narrow range of the angular position.
In contrast to this, in U.S. Pat. No. 5,861,745, as shown in FIGS. 9A to 9D, an angular position sensor which is constructed so that semicircular auxiliary stators 110 are installed on both sides of a magnetism detection element 100 so that the air gap between a cylindrical magnet 120 and the auxiliary stators 110 is uniform. In this angular position sensor, the direction of the magnetic flux passing through the magnetism detection element is always perpendicular to the direction of magnetism detection by the auxiliary stators, and the amount of the magnetic flux passing between the auxiliary stators 110 changes with rotation. Thus it provides a linear output over a wider range of the angular position.
However, in the above angular position sensor, since a magnetic circuit is formed in point symmetry, the positional relation is the same except that the poles of the magnet 120 are reversed when it is rotated 180xc2x0. Accordingly, as shown in FIGS. 9A to 9D, the maximum or minimum points (peak points) appear at every 180xc2x0. That is, a range in which the output is linear is a maximum of 180xc2x0 (between xe2x88x9290xc2x0 and 90xc2x0), and it is impossible to provide a linear output over a wider range than this. Practically speaking, since the characteristics in the vicinity of the peak points) are not perfectly linear, the actual linear range is more limited to 160xc2x0.
The present invention overcomes the above drawbacks, and has as an object to provide an angular position detection device which is capable of establishing linear characteristics of an output of a magnetism detection element over a range wider than 180xc2x0, thereby improving characteristics of angular position detection.
The angular position detection device according to the present invention comprises one or two magnets for generating a magnetic field and one or more magnetism detection elements disposed in the magnetic field for detecting magnetism. The magnets are installed in a rotor and the magnetism detection elements are installed in a stator, or vice versa. The stator is disposed in the inside of the rotor, which is fixed to a rotary shaft of an object to be detected. The stator or rotor in which the magnets are installed has a primary yoke that forms a magnetic circuit of the magnets, and is configured so that the intensity distribution of the magnetic field is asymmetric with respect to the center of rotation of the rotor. On the other hand, the stator or rotor in which the magnetic detection elements are installed has a pair of auxiliary yokes, and the magnetic detection elements are disposed in the gap provided between the pair. The stator and the rotor are configured so that the air gap between them varies gradually in the circumferential direction.
According to this construction, since the point symmetry in the magnetic circuit does not hold, the angular interval between the peak points in output characteristics of a magnetism detection element is larger than 180xc2x0 and thus a linear range wider than 90xc2x0 can be provided. As a result, characteristics of detection of the angular position can be improved.